Inflammation contributes at each stage in the development of clinically significant atherosclerosis. The initiation and progression of atherosclerosis is decreased in regions of steady flow associated with high laminar shear stress, compared to regions of turbulent and low flow. This finding has yielded the concept that steady laminar flow is atheroprotective. The major hypothesis of this proposal is that signal transduction events in endothelial cells (EC) elicited by steady laminar flow limit atherosclerosis by decreasing inflammation. Our laboratory has focused on regulation of the mitogen activated protein kinases (MAPK) by flow. During the previous grant period we tested the hypothesis that understanding the mechanisms by which flow regulates c-Jun N-terminal kinase (INK) activation by cytokines will provide insight into the atheroprotective mechanisms induced by flow. The hypothesis was validated as we discovered two molecules - thioredoxin and Big MAPK-1 (BMK1) - that were regulated by flow and inhibited INK activation by TNF. Our lab was first to show that flow activated BMK1 in EC. The importance of BMK1 in EC function has been validated by the findings that BMK1 null mice display defective EC morphology and blood vessel formation leading to embryonic lethality. Based on preliminary data we propose a mechanotransduction pathway for BMK1 activation that involves platelet endothelial cell adhesion molecule (PECAM1)-SHP2 phosphatase-Gabl adaptor protein-MEKK3-MEK5-BMKl. To define the extent to which this pathway mediates the atheroprotective effects of steady laminar flow we propose four aims. 1) Show that PECAM1 is a mechanosensor necessary for flow-induced BMK1 activation. 2) Show that Gabl translocation and tyrosine phosphorylation are required for flow-mediated BMK1 activation. 3) Show that Phox and Beml (FBI) domain-containing proteins specify assembly of a MEKK3-MEK5 signaling complex by flow that activates BMK1, but not INK.. 4) Characterize the effect of endothelial-specific BMK1 deletion or activation on atherosclerosis in ApoE knockout mice. These studies should provide insight into mechanisms by which flow inhibits vascular inflammation and facilitate development of new therapeutic approaches to limit atherosclerosis.